Commonly computers or microcomputers are required to control a colon display on a raster scan color display unit such as a color cathode ray tube. In such a raster scan display unit each scanning line comprises a succession of pixels and it is necessary to supply analog input signals to the red, blue and green color inputs of the cathode ray tube at the pixel frequency in order to establish the correct color composition for each pixel. The color composition required for each pixel may be indicated by a numerical value stored in a pixel memory or bit map. Values from this pixel memory are read at the pixel frequency and must then be converted into appropriate analog signal values to each of the different color inputs for the cathode ray tube. Color look-up tables are known for this purpose. In this case a combination of red, blue and green color values is derived from a look-up table for each pixel value. Due to the high pixel frequencies normally used in a raster scan display problems arise in deriving red, blue and green color values from a color look-up table embodied in an integrated circuit memory device. Previous devices capable of operating at high pixel frequencies have involved many separate components at relatively high cost and with substantial power comsumption.
It is therefore an object of the present invention to provide a color graphics control system in which stored pixel values may be used to generate a succession of electrical signals representing respective color inputs for a raster scan color display unit at high pixel frequencies with low cost and low power consumption.
It is a further object of the invention to provide an improved color graphics control system which may be incorporated in a single integrated circuit chip.